How to add a image to a textured wall?
The basic way to do it is this one:


Create your graffiti with a transparent background (on Photoshop or directly on Blender, in that case create a new image with a 100% transparent alpha background).
Mix your brick image and your graffiti image with a Mix Shader.
Plug the Alpha output of the graffiti Image Texture node into the Factor input of the Mix Shader so that the transparent surface of the graffiti image will work as a mask.
You can create a second UVMap to manipulate the graffiti coordinates, size or rotation, independently from the brick texture: In the Properties panel > Data > UVMaps, add a new one with the +, then unwrap again your object, then, before the Image Texture of the graffiti, create an Input node > UV Map, and choose this second unwrap as the UV coordinates.